A crane safety apparatus has been proposed (in Japanese Patent Publication No. 56-47117) in which various operation parameters (such as boom length, boom angle, outrigger extension, and jibbing) of a crane are detected by sensors, a digital memory storing rated loads for various operation states determined by the specification of the crane is accessed to retrieve the rated load specific to the detected operation parameters, the retrieved rated load is compared with the actual load, and a warning is issued when the actual load reaches a value near the rated load or the crane is automatically stopped when the actual load reaches the rated load.
A conventional crane safety apparatus does not have a function of correctly indicating a hook lifting distance. The hook lifting distance is known from the present state of the hook structure. However, there is no practical method of correctly knowing the length of a hook structure hung from the boom or jib top by the released rope. Furthermore, an apparatus has not been proposed which schematically displays a hook structure on a display within a target or operation range fixedly set by an operator, and allows the operator to monitor the hoisting and lowering operation of the hook structure.